1. Field of the Invention
The present invention relates to a power series predistorter that compensates for distortion produced by a power amplifier used in a mobile phone terminal and a base station, for example, and a control method thereof.
2. Description of the Related Art
Microwave power amplifiers used in base stations or terminals of mobile telecommunication systems are required to achieve high efficiency in order to reduce the power consumption or size thereof. In general, the power added efficiency of a power amplifier becomes higher as the output power thereof becomes closer to the saturation output power, and thus, it is desirable that the power amplifier operates in that region. However, if the power amplifier operates with output power close to the saturation output power, the distortion component produced by the power amplifier increases. Since the power amplifiers used in the base stations and terminals have to achieve a predetermined adjacent channel leakage power ratio outside the signal band as defined radio systems regulations, the power amplifiers are operated with sufficient output backoff. Therefore, the power added efficiency of the amplifiers becomes poor. Thus, in order to operate the power amplifier with output power close to the saturation output power to achieve higher power added efficiency, the distortion component produced by the power amplifier has to be suppressed.
Various nonlinear distortion compensation methods for suppressing the distortion component have been developed. One of such nonlinear distortion compensation methods is a predistortion method. The predistortion method is to previously add a predistorted distortion component that cancels the distortion component produced by the power amplifier to the input signal. The distortion component produced by the power amplifier can be cancelled by configuring the predistorted distortion component to be previously added (referred to as predistortion, hereinafter) to be equal in amplitude and opposite in phase to the distortion component produced by the power amplifier.
FIG. 18 shows a configuration of a typical power amplifier incorporating a conventional power series predistorter. In this example, a power series predistorter 100 comprises a divider 1, a linear transmission path 2, a distortion generation path 3, a combiner 4, a distortion detector 9 and a vector adjustment controller 10. The distortion generation path 3 includes a distortion generator 31 and a vector adjuster 32. A transmission signals x(t) of base band is input to the divider 1, which distributes the transmission signal x(t) of base band to the linear transmission path 2, which is constituted by a delay unit 21, and the distortion generation path 3. In the distortion generation path 3, the distortion generator 31 generates a third-order predistortion component expressed as |x(t)|2x(t) (referred to as D3, hereinafter), for example, in accordance with the input transmission signal x(t) of base band, and the vector adjuster 32 adjusts the amplitude and phase of the predistortion component and inputs the adjusted predistortion component to one terminal of the combiner 4. To the other terminal of the combiner 4, the transmission signal x(t) of base band delayed by the delay unit 21 is input. The transmission signal of base band and the predistortion component are combined together by the combiner 4, the combination signal is passed to a frequency converter 5 as an output of the predistorter 100 and converted into a transmission signal of radio frequency, and the transmission signal of radio frequency is amplified by a power amplifier 6.
The output signal of the power amplifier 6 is transmitted to an antenna element (not shown) via a divider 7, and the divider 7 passes part of the output signal to a frequency converter 8. The frequency converter 8 converts the output signal from the divider 7 into a down-converted signal of base band and passes the down-converted signal of base band to the distortion detector 9. The distortion detector 9 detects the distortion component of the down-converted signal that is produced by the power amplifier 6 and passes the detected distortion component to the vector adjustment controller 10. The vector adjustment controller 10 controls the vector adjuster 32 in the distortion generation path 3 to adjust the amplitude and phase of the third-order distortion component in such a manner that the third-order component in the output signal of the power amplifier 6 detected by the distortion detector 9 is minimized. Controlling the amplitude and phase in the vector adjuster 32 in this way can cancel the distortion component produced by the power amplifier. It is known that the conventional predistorter can compensate the symmetric third-order distortion component which is not frequency dependency.
However, if the power amplifier 6 operates around the saturation output power, the conventional predistorter described above may not be able to achieve higher distortion compensation. This is because a memory effect occurs in the nonlinear characteristics of the power amplifier, as described in W. Bosch and G. Gatti, “Measurement and simulation of memory effects in predistortion linearizer,” IEEE Trans. Microwave Theory Tech., vol. 37, pp 1885-1890, December 1989 (referred to as non-patent literature 1, hereinafter). The memory effect is produced by passing a distortion component through a band-pass filter, and causes to impart a frequency characteristic to the distortion component produced by the power amplifier 6. This results in uneven frequency characteristics on the higher and lower sides of a signal band 400 after the predistortion compensation (indicated by a dashed line) unlike the frequency characteristics before the predistortion compensation (indicated by a solid line) as shown in FIG. 19, for example. The conventional predistorter cannot compensate for such frequency dependency.
A technique to cope with the distortion compensation with the frequency dependency is disclosed in Japanese Patent Application Laid-Open No. 2002-57533 (referred to as patent literature 1, hereinafter). FIG. 20 is a diagram illustrating the prior art disclosed in the patent literature 1. The prior art differs from the power series predistorter described above with reference to FIG. 18 in the configuration of the distortion generation path 3. The distortion component generated by the distortion generator 31 is split into a low frequency band distortion component and a high frequency band distortion component via a low pass filter 351 and a high pass filter 352. The amplitude and phase of the distortion of the low frequency band distortion component are adjusted by a vector adjustment circuit 321, and the amplitude and phase of the distortion component of the high frequency band distortion component are adjusted by a vector adjustment circuit 322. In this way, the distortion component generated by the distortion generator 31 is split into two frequency bands by two filters, and the frequency characteristics of the distortion components of the respective frequency bands are adjusted, thereby suppressing the frequency dependent distortion compensation.
According to the compensation method of reducing the frequency dependency of the distortion, plurality of filters are used to split the frequency dependency of the distortion component. However, since frequency characteristics of the filter generally cannot have infinite rise and fall at the ends of the pass band thereof, an overlap or drop occurs at the boundary between the pass bands of the filters, so that a discontinuity occurs in the amplitude frequency characteristic and the phase frequency characteristic. Therefore, there is a problem that, in the case where a modulated signal having a continuous spectrum is input, a continuous amplitude frequency characteristic and phase frequency characteristic cannot be imparted to the predistortion component because of the variations of the frequency characteristics of the filters.